A correlation of the viscosity of hydrocarbon systems with pressure, temperature and composition

Abstract

The work reported herein presents a new equation for the accurate prediction of the viscosity of several pure paraffin hydrocarbons and nitrogen. The equation applies reliably to both liquids and gases, and is cubic in viscosity. It involves temperature, pressure and six empirical constants of the material. The equation is similiar in form to van der Waals' equation of state. It differs from other equations for viscosity, in that it does not include the density of the pure material at the pressure and temperature of interest. It was applied to methane, ethane, propane, n-butane, n-pentane, n-hexane and nitrogen, with an average absolute deviation of 1.9%, based on 1,006 data points described in the literature and reported by 14 authors. When this equation is extended to mixtures by assuming a simple relation between its constants and the properties of the components, a correlation is obtained with an average absolute deviation of 9.6%. This compares with 16.0% for the deviation on the same samples resulting from the application of the only other method relating viscosity to temperature, pressure and composition. A second equation for mixtures was developed using regression analysis applied to pressure, temperature, composition, mixture density as calculated by the Alani equation, characteristics of the heptanes-plus fraction, pure component viscosities and average moleculare weight of the oil. The equation has 20 terms and has an average absolute deviation of 7.70% based on 3,350 experimentally determined viscosities obtained on 829 complex liquid systems. Both equations are applicable to hydrocarbon systems containing impurities of nitrogen, carbon dioxide and hydrogen sulfide and a heptanes-plus fraction. Viscosity measurements, using a specially designed rolling ball viscometer, were made on n-hexane from 600 to 10,000 psia and 70-280°F and correlated with the proposed equation for put materials with an average absolute deviation of 0.93%.